On Indigo. 431. 
point of a knife; they are crystals of sub- 
limed indigo. 
Precipitated and sublimed indigo appear by 
the chemical tests to be constituted of the 
same elements; and no doubt is entertained 
that they present the pure colouring matter 
of indigo in its most concentrated form. 
Three chemists have published analyses of 
pure indigo within the last three years; 
namely, Drs. Thomson and Ure, and Mr. 
W. Crum, all of Glasgow. The same plan 
was adopted by all three, namely, burning 
a small given portion of indigo in contact 
with the black oxide of copper in ‘green 
glass tubes. The indigo being finely divided 
and intimately diffused through a comparative- 
ly large portion of the oxide, heat is applied 
sufficient to burn the carbone and hydrogen 
of the indigo, and to liberate the azote; 
hence from the quantities of carbonic acid 
and azote produced, and the loss of weight 
which the oxide sustains, the constituents of 
indigo are inferred. The results are below: 
Dr. Thomson* Dr. Uret Mr. Crumf{ 
Carbone 40.39.........006 TST iksccadees 73.22 
Azote SAG: jst bedanae 10.—.....ceeeeee 11.26 
Oxygen 46.15.......0000 16 O55. csnety. GO 
Hydrogen 0 — .eseoecssees 4.38... seocccese 2.92 
100 100 100 
© Annal. of Philos. June, 1820-——fIbid. Feb, 1823. 
+ Philos, Trans, 1822. 
